The present invention is directed to the detection of coherent signals, in general, and more particularly, to a system and method of determining the frequency of a coherent signal detected from an incoming signal using a frequency estimate of the coherent signal to expedite the processing of a time segment of sampled data signals associated with the coherent signal.
In systems utilizing Doppler techniques for measuring parameters, such as a laser Doppler velocimeter (LDV) or a LIDAR, for example, coherent bursts of Doppler frequency shifted echo or return signals that are received intermittently and for brief durations are processed to measure each associated parameter. Not only is the detection of these coherent bursts from the incoming or received signal important, but an accurate and expeditious determination of the frequency of the detected burst is equally important. In an LDV or LIDAR system, for example, the signal frequency of the coherent burst is linearly proportional to a component of a velocity parameter being measured. This Doppler shifted frequency may range over several orders of magnitude approaching one gigahertz. Accordingly, the accurate and reliable frequency measurement of such coherent bursts with a low signal-to-noise ratio (SNR) is critical to accurate velocity measurements.
Because of the frequency ranges or bandwidths involved with such parameter measurements, real time signal processing of the coherent bursts is currently limited to crude frequency resolution. In general, a front end real time coherent burst detector, which may be of the type described in U.S. Pat. No. 4,973,969 or U.S. Pat. No. 5,289,391, for example, is coupled with a post-processor for processing the burst signals to obtain a high resolution frequency measurement. The coherent burst detector detects the coherent pulse in the incoming signal, collects data samples of the incoming signal over the time segment associated with the detected burst, and triggers the post-processor to process the collected data samples to determine the burst frequency. In order for the post-processor to determine the frequency of the burst accurately, it needs to process a large number of data samples with a commensurately sized spectral transform algorithm. However, speed of processing greatly diminishes as the size of the spectral transform increases to meet greater accuracy requirements creating longer processing times per burst. Accordingly, if a subsequent burst occurs during the processing of a burst, it may not be processed.
Currently, update rates for tracking the coherent bursts dictate the processing time allocated to process a burst signal to determine the frequency thereof at a specified accuracy or resolution. Therefore, update rates and accuracy requirements have a tendency to conflict with one another. That is, the processing time for each burst is set to ensure the processing of the bursts as they occur without loss of substantive burst information. As the update rate increases, the specified processing time decreases. On the other hand, the accuracy or resolution of the frequency measurement is not diminished. Accordingly, the post-processor has to maintain accuracy, but with a shorter processing time. Current coherent burst signal processing systems are unable to resolve this conflict.
Accordingly, what is desired is a system and method for measuring frequency of the coherent bursts at speeds and accuracy commensurate with the current and future demands of update rates and resolution. A processor that can process the sampled data of a coherent burst signal quickly and accurately will ensure that a minimum number of intermittent bursts are missed while waiting for the completion of a burst signal frequency measurement.
In accordance with one aspect of the present invention, a method of determining the frequency of a coherent signal from an incoming electrical signal comprises the steps of: generating a time sequence of sampled data signals from the incoming electrical signal; detecting the coherent signal in the time sequence of sampled data signals and generating a frequency estimate thereof; and determining the frequency of the detected coherent signal based on a function of the frequency estimate and a time segment of sampled data signals associated with the coherent signal.
In accordance with another aspect of the present invention, a system for determining the frequency of a coherent signal from an incoming electrical signal comprises: means for generating a time sequence of sampled data signals from the incoming electrical signal; means for detecting the coherent signal in the time sequence of sampled data signals and generating a frequency estimate thereof; and means for processing a time segment of the sampled data signals associated with the detected coherent signal and the generated frequency estimate to determine the frequency of the detected coherent signal.